I would say you loose your ability to order units. You can't go back and play a command card at that point. I had this situation today. Knowing that it is a 7-lore card, your opponent has to have at least 4 Lore ot dispel it. So I only used it once I knew he couldn't counter it.

Question: What happens if my Assault (or any other Lore card played "in lieu of a Command card" is foiled or dispelled by my opponent? Do I get to play a command card instead, or do I remain with no activated units for the turn?

Answer: You get to play a Command card instead (but still must discard your foiled/dispelled Assault, and cannot play another Lore card this turn).

I agree that it is counting as a command card so you've basically been trumped. That's an extreme version of those lore spells that negate movements of opponents, forcing them to go back to their starting place. They're not allowed to move someone else afterwards...

Extreme example that turned out nicely for me:
Opponent plays Leadership(?) card, giving a single unit an extra die cast. My opponent simultaneously plays a card that increases his die cast even more...extra powerful. Very scary!

He then moves his mega-fighter next to my unit, ready to wreak havoc upon me. I cast a lore which forces him to go back to his original location, and not fight or move. Whimpering begins.

He wasted not only his command card but a spell and the lore that it cost to cast it. Nice for me, eh?